Overhaul input validation of sparse() function. * sparse.cc (Fsparse): Decode input type and identify floating point inputs. If input is of single type, emit new warning "Octave:sparse:double-conversion". If input is neither floating point or logical, then call err_wrong_type_arg() for pretty error message. Rename temporary variable 'k' to "argidx" for clarity. New temporary variable "arg" to increase readability of code. Add FIXME note about unreachable code due to behavior of get_dimensions(). * sparse.cc (Fissparse): Turn off warning about double-conversion temporarily for test which has single input. * warning_ids.m: Add description for new warning ID "Octave:sparse:double-conversion". * mk-sparse-tst.sh: Redo BIST tests for sparse() construction.